Changes in microbial and soil properties following compost treatment of degraded temperate forest soils

Abstract

The long-term effect of compost treatment on soil microbial respiration, microbial biomass carbon (C mic) and biomass nitrogen (N mic), soil organic carbon (SOC), and soil total nitrogen (STN) was studied in six degraded forests, Lower Saxony, Germany. The study was conducted in mature beech ( Fagus sylvatica L.), pine ( Pinus sylvestris L.) and spruce ( Picea abies Karst.) forests on silty soils at Solling and on sandy soils at Unterlüß. Mature compost from separately collected organic household waste was applied for soil amelioration at an amount of 6.3 kg m −2 on the soil surface. After 2 years, soil samples were taken from the control and compost plots and were separated into >2 and <2 mm fractions of the O-horizon and into mineral soil intervals from 0–5, 5–10, and 10–20 cm depths. The original compost had a pH of 7.5, high inorganic salt content, low organic C content, narrow C-to-N ratio, and low microbial activity and biomass. Compost significantly reduced the microbial respiration per mass unit in the O-horizons >2 mm by 17% and in the O-horizons <2 mm by 25%. C mic and N mic decreased significantly by 22 and 23% in the O-horizons <2 mm and by 35 and 28% in the O-horizons >2 mm, respectively. Our estimates suggest that the reduction in microbial respiration and biomass in the O-horizons resulted partly from the mixture of compost and the O-horizons. The average loss of 1.2 kg m −2 organic matter may have also contributed to the reduction in microbial biomass and respiration in the O-horizons of the compost plots. However, it is not clear whether the decomposition of the original organic matter in the O-horizons was increased by the compost application. In the mineral soils, the compost treatment caused significant increases in microbial respiration, C mic and N mic by 14–21% at 0–5 cm and by 14–23% at 10–20 cm depth. Although not significant, a similar trend was found for the 5–10 cm depth. Increased release of nutrients and dissolved organic matter (DOM) could have promoted microbial growth and activity in the mineral soils. The significant increase in STN and the narrowing C-to-N ratio indicate that the investigated forest soils were not N-saturated. This field study suggests that superficial application of compost from separately collected organic household waste increase microbial activity and biomass in the mineral soil by release of nutrients from the O-horizon to the mineral soil.